240 SOME MINUTE ANIMAL PARASITES 



The exact time at which reproduction of the Myxo- 

 sporidia occurs is uncertain, but it may be stated 

 generally that cold weather is favourable to spore 

 formation, while warm weather tends to prolong the 

 purely vegetative phase of the organism's existence. 

 The details of the processes leading up to spore 

 formation vary slightly in the different genera of the 

 group, but there is a general type of structure that 

 prevails in all. Within the body of the trophozoite 

 one or more portions are differentiated for reproduc- 

 tion. Each portion possesses a definite nucleus, and 

 its cytoplasm gradually becomes distinct from that 

 of the rest of the body. These specialized portions 

 are the pansporoblasts, or spore progenitors. Some- 

 times they occupy a fixed position in the cell; at other 

 times, especially when more than two are present, 

 their positions are very varied. Zonal arrangement 

 is known in some. 



Each pansporoblast at first is uninucleate, but its 

 nucleus divides rapidly to form either two or several 

 nuclei, so that the pansporoblast becomes multi- 

 nucleate. Earch division or sporoblast secretes a 

 cell wall around itself. This cyst wall, or sporocyst, 

 is distinctly bivalved and a clearly marked sutural 

 line is present. The spore is thus produced. It, 

 too, is multinucleate. Two nuclei control the forma- 

 tion and regulate the movements of the valves. 

 They are variously termed the parietal, valvular, or 

 sporocyst nuclei. Within the sporocyst two vacuoles 

 develop, or in the Chloromyxidse four are produced. 

 These may lie side by side, or may be at opposite 

 poles of the spore. Their positions are points used 



